Numerous pay-TV channels currently fall victim to fraud. In particular, pirate cards are frequently used to view their channels. This invention proposes a new system for transmission of image decryption keys (or of the image itself) which offers numerous advantages: the system is relatively simple to implement and can react quickly if pirate cards should appear (flexibility).
If a pirate card is obtained, it is possible to find out from the exterior (i.e. just by observing its operation) what secrets it holds, which may possibly be used to find out from which real card it obtained these secrets, but especially to quickly disable all the pirate cards without disabling the legitimate cards. This is known as traitor tracing and in particular black box (traitor) tracing. Note that the invention proposed is extremely efficient and secure compared with the other systems proposed in the cryptographic literature (see references). Note also that this invention is not limited to television: the method can also be used whenever the same content must be transmitted to several authorised receivers.
The new method is characterised by very reasonable rates which are compatible with the speed limitations imposed by the communication channels. In addition, it stands out from other methods due to the very short length of the data K which is transmitted in real time in order to access the protected content: this length can be as short as just 64 bits.
The invention therefore concerns a method to make the same information (Kc) available to several receivers belonging to a group (G) of receivers, each receiver storing information (SAi) specific to it, characterised in that it comprises the following steps:                define a relation Kc=f(K, bi, SAi) where (f) is a given function, (K) is information common to all the receivers, and (bi) is information different for each receiver and for each value of the information (K);        enable each receiver to access information (bi) before making (Kc) available; and        transmit the information (K) to all receivers, just before making (Kc) available;        so that each receiver can calculate information (Kc) using said relation.Advantageously, the function (f) is such that knowing a (bi) and a (SAi), no algorithm is known which could be used to obtain the information (Kc) in a realistic time and with non negligible probability, when the information (K) is not known.Advantageously, function f is such that, knowing a certain number of (b1 . . . bn) for a certain subgroup (G′) of receivers, no algorithm is known which could be used, before knowing the current K, in a realistic time and with a non negligible probability, to produce a valid pair (bi, SAi) with a legitimate (SAi), i not being one of the receivers 1 . . . n of (G′).        
Advantageously, the function f has the format:f(K,bi,SAi)=bi⊕EK(SAi)where EK is a function depending on information (K) and where ⊕ designates a group law.Advantageously, function (EK) is a cryptographic encryption function and (K) a secret key used by this function.Advantageously, the values (bi) are sent encrypted with a key (Ki) specific to each receiver of a certain group (G) of receivers.Advantageously, each value (SAi) is a secret value known by the receiver of index i.Advantageously, each (bi) consists of two values b1i and b2j and equally the information specific to each receiver consists of two values SAi et SAj, such that each receiver, identified by the pair of indices (i,j), combines the corresponding values b1i and b2j with the values SAi and SAj to calculate values Kc1 and Kc2 using said relation, which are in turn combined to access the information KC.Advantageously, the information Kc is a key used to decrypt a digital content such as a television image.Advantageously, the information Kc can be used for several minutes by the receivers, the information K is sent several seconds in advance and the values bi are sent regularly, starting several days in advance.Advantageously, certain receivers find at least some of their values bi in a list of values prestored in the receivers.The invention also concerns a portable receiver object belonging to a group (G) of portable objects and comprising information processing means and information storage means, the storage means storing information (SAi) which is specific to the portable object and a given function (f), characterised in that it comprises:
means to obtain access to information (bi) different for each portable object of the group (G) and for each value of the information (K); and
means to calculate information (Kc) using a relation Kc=f(K, bi, SAi) where K is information common to all the portable objects and transmitted to them.
Lastly, the invention concerns a transmitter device to make the same information (Kc) available to several receivers belonging to a group (G) of receivers, each receiver storing information (SAj) specific to it, characterised in that it comprises:
                calculation means designed to calculate information (bi) using a relation Kc=f(K, bi, SAi) where (f) is a given function, (K) is information common to all the receivers and information (bi) is information different for each receiver and for each value of the information (K); and        transmission means designed to transmit to each receiver, a certain time before making (Kc) available, the information (bi) associated with it, and to transmit information (K) to all the receivers, just before making (Kc) available.        